1. Field of the Invention
The present invention relates to a malfunction determining method of a heating unit, a heating unit using the malfunction determining method, an image fixing device including the heating unit, and an image forming apparatus including the image fixing device.
2. Description of the Related Art
In a conventional image forming apparatus such as a copier, a printer, etc., after one or more toner images are formed on a recording paper, it is necessary to carrying out an image fixing process of dealing with the toner images on the recording paper by an image fixing device so as to fix the toner images on the recording paper. In general, the conventional image forming apparatus carries out the image fixing process to deal with hot-melt toner by a combination of heating and pressurization. As a result, it is very important to the quality of fixing the toner images whether the temperature of a heating unit in the image fixing device is normal. Once the temperature of the heating unit is not normal, it may affect not only the quality of fixing the toner images but also the safety of the whole image forming apparatus. Therefore it is very important that whether there is malfunction in the heating unit can be determined timely and accurately.
In Japanese Patent Laid-Open Publication No. 2007-329041, an image fixing device having a malfunction determining function is disclosed. In this image fixing device, during a time period from the time when a heating unit begins to be heated to the time when the temperature of the heating unit reaches a predetermined image fixing temperature, it is determined according to the temperature increase rate of the heating unit whether there is malfunction in the heating unit. In particular, in a case where the temperature increase rate of the heating unit is in a range of 10° C./s to 13° C./s, it is determined that there is no malfunction in the heating unit (i.e. the heating unit is in a normal state); in other cases, it is determined that there is malfunction in the heating unit (i.e. the heating unit is not in a normal state).
In Chinese Patent Laid-Open Publication No. 1936727A, an image fixing device having a malfunction determining function is also disclosed. In this image fixing device, during a time period from the time when a heating unit begins to be heated to the time when the temperature of the heating unit reaches a predetermined stand-by temperature, it is determined according to the temperature rise time of the heating unit whether there is malfunction in the heating unit.
It is apparent that, in the above-mentioned two conventional techniques, the same technical approach is employed to carry out the malfunction determination no matter what temperature range the heating unit is currently in.
However, in a situation where the heating unit begins to be heated from a low temperature stage, when the temperature of the heating unit is still at the low temperature stage, i.e., the temperature of the heating unit has not reached a mid-to-high temperature stage yet, the temperature increase rate of the heating unit is relatively slow. Especially at the beginning of a time period where the heating unit is heated, the temperature increase rate of the heating unit is very slow; that is, as shown in FIG. 1, the temperature increase rate of the heating unit can reach a relatively stable state only after a predetermined time period where the heating unit is heated continually. As a result, in a circumstance where the temperature of the heating unit is at the low temperature stage, if the malfunction determination is carried out by using the temperature increase rate as described in the Japanese Patent Laid-Open Publication No. 2007-329041, it is determined that there is malfunction in the heating unit if the temperature increase rate is not in the range of 10° C./s to 13° C./s. It is apparent that this may cause a severe problem of false reporting, and cannot satisfy an accuracy demand of malfunction determination. Due to the property that the temperature increase rate of the heating unit is not stable at the low temperature stage, it is very difficult to choose a proper parameter by which the malfunction determination can be carried out timely and accurately.
Furthermore, as shown in FIG. 1, with the continuance of heating applied to the heating unit, the temperature increase rate of the heating unit speeds up gradually. After the temperature of the heating unit reaches the mid-to-high temperature stage, if there is malfunction in the heating unit and the malfunction cannot be detected timely, the recording paper may be burned with the increase in temperature, and eventually other parts of the heating unit or the image forming apparatus may be damaged. As a result, in a case where the temperature of the heating unit is at the mid-to-high temperature stage, timeliness of malfunction determination of the heating unit is strongly required. However, in a situation where the temperature of the heating unit is at the mid-to-high temperature stage, if the malfunction determination is carried out by using the temperature rise time as described in the Chinese Patent Laid-Open Publication. No. 1936727A, the timeliness demand of the malfunction determination cannot be satisfied.
It is apparent that, in the above-mentioned two conventional techniques, since the property of temperature increase and the demand of the malfunction determination of the heating unit are different as the temperature ranges in which the temperature of the heating unit may be are different, if the malfunction determination is carried out by employing the temperature increase rate or the temperature rise time at the middle-temperature stage and the high-temperature stage, the accuracy and the timeliness of the malfunction determination may be affected.